For the most part research in medical history has attended to the advances that medicine has made over the centuries, thus drawing attention to the changes that have occurred. I draw attention to the aspects of medicine that have not changed, to certain problems that have remained invariable. The answers have changed but the problems have remained the same. I identify four such problems: What is the disease? What is its cause? What is the treatment? and How good is the evidence? Using concrete examples of diseases and of research into their nature I show that these problems illustrate the old expression, "The more that things change, the more they remain the same." My studies involve not only the history of medicine but also the meaning and nature of diagnosis, causality, classification, evidence, the scientific method. The research involves history of medicine, philosophy of science, and the history of ideas.